ACS ISR 2012-04 contains plots of likely minimum sky backgrounds as a function of exposure time. Observers should determine if their sky backgrounds are likely to be less than 20 electrons. If this is the case, they should consult the ACS webpages for details of the post-flash capability.

At wavelengths longer than ~8000 ┼, internal scattering in the HRC CCD produced an extended PSF halo. Only a small number of observations were affected because WFC was mostly used at these wavelengths. The WFC CCDs were treated with a front-side metallization that eliminates the large angle, long wavelength halo problem for wavelengths less than 9000 ┼. For observations of red targets with the F850LP refer to Section 9.3.2.

Table 5.5 contains the detection limits in Johnson-Cousins V magnitudes for unreddened O5 V, A0 V, and G2 V stars, generated using the ETC. WFC and HRC values used the parameters CR-SPLIT=2, GAIN=2, and a 0.2 arcsecond circular aperture. For the SBC, a 0.5 arcsecond circular aperture was used. An average sky background was used in these examples. However, limiting magnitudes are sensitive to the background levels; for instance, the magnitude of an A0 V in the WFC using the F606W filter changes by ▒0.4 magnitudes at the background extremes. Figure 5.8 shows a comparison of the limiting magnitude for point-sources achieved by the different cameras with a signal to noise of 5 in a 10 hour exposure. Figure 5.9 shows a comparison of the time needed for extended sources to attain ABMAG=26.

Figure 5.7: HST total system throughputs as a function of wavelength. The plotted quantities are end-to-end throughputs, including filter transmissions calculated at the pivot wavelength of each broad-band filter.

Figure 5.8: HST Limiting Magnitude for point sources in 10 hours, as a function of wavelength. Point source limiting magnitude achieved with a signal to noise of 5 in a 10 hour long exposure with optimal extraction.

Figure 5.9: HST Limiting Magnitude for extended sources in 10 hours, as a function of wavelength.

Chapter 10 contains plots of exposure time versus magnitude for a desired signal-to-noise ratio. These plots are useful for determining the exposure times needed for your scientific objectives. More accurate estimates require the use of the ACS ETC (http://etc.stsci.edu/).